In 2009, Sharon Kujawa and M. Charles Liberman introduced to the auditory neuroscience field the fascinating phenomenon of hidden hearing loss—a circumstance in which a brief noise exposure causes a temporary elevation of hearing thresholds, resulting in dramatic, detrimental, and permanent physiologic and histologic consequences to the peripheral auditory system. The response of the cochlear nerve to suprathreshold acoustic stimulation was significantly diminished in these animals with otherwise normal hearing thresholds. Moreover, the synaptic populations electrophysiologically connecting the inner hair cells to auditory neurons were decimated. Notably, this reduction in peripheral signal has since been suggested to result in central gain and contribute to the development of tinnitus and hyperacusis, among other auditory conditions (J Neurophysiol. 2014;111:552 http://bit.ly/2e7pgR4;Neuron 2016;89:867 http://bit.ly/2dsN1b9). The investigators demonstrated this effect in multiple mammalian species and hypothesized early on that this phenomenon likely occurs in humans as well (J Assoc Res Otolaryngol. 2011;12:605 http://bit.ly/2dsLXnz). It appears that they are correct.
Independently, the Melcher and McAlpine groups found that normal hearing subjects with tinnitus had reduced wave I amplitudes of their auditory brainstem response (ABR), representing the summed activity of the cochlear nerve. Evidence for perceptual consequences of hidden hearing loss has also been mounting. Bramhall and associates reported that reduced ABR wave I amplitude was associated with impaired speech-in-noise performance, while Liberman and colleagues found that young adults with substantial noise exposure history had poorer word-recognition-in-noise performance and physiologic evidence of auditory neuronal loss. Intriguingly, this cohort also demonstrated significantly heightened sound annoyance and avoidance symptoms, hinting at the role of auditory neuronal degeneration in the development of hyperacusis.
So what can clinicians take from these remarkable findings? If neuronal loss from advancing age or noise trauma plays an instrumental role in a variety of auditory pathologies, what interventions can be provided to symptomatic patients? Animal studies continue to provide further insights into these clinical questions. In recent decades, successful promotion of auditory neuronal survival via neurotrophin delivery, gene therapy, and electric stimulation, among others, has been thoroughly demonstrated in animal models. Of particular note, the O'Leary and Liberman groups independently described a significant attenuation of the impact of noise trauma to ABR wave I amplitudes and cochlear synaptic populations with round-window delivery of neurotrophins.
Currently, there are few, if any, roadblocks to the delivery of neurotrophins to the human inner ear; otolaryngologists routinely provide medication to the cochlea via transtympanic injection. Moreover, a host of gel formulations has been developed to provide chronic delivery of therapies, and these materials could provide weeks to months of neurotrophic support to the spiral ganglia. Viral-mediated inner ear gene therapy has already been trialed in humans, and accordingly, the road to study the impact of neurotrophin production from transfected cochlear cells has been well-paved.
Although auditory neuronal stimulation and tinnitus suppression via inner ear electric stimulation has long been well-established, providing patients with safe and stable current to the cochlea remains a substantial technical obstacle (Curr Opin Otolaryngol Head Neck Surg. 2015;23:382 http://bit.ly/2dsJdq5). Recent developments of innovative implantable auditory prostheses may inspire novel methods of the delivery of electricity to the spiral ganglia.
These and other lines of research strongly suggest the role of the hidden hearing loss phenomenon in the pathophysiology of a myriad of troublesome auditory symptoms and, more importantly, the need to translate landmark animal studies to clinical trials on the diagnosis and treatment of hidden hearing loss.